The development of low cost, renewable energy capabilities is critical for future air, terrestrial and space transportation, as well as for distributed electric power generation. H2 can replace fossil fuels for the production and storage of energy. H2 can be produced from various resources: renewables, nuclear energy, and coal. High efficiency and low emissions are achieved through use of fuel cells. H2 fuel cells can power cars, boats and aircraft. Its generation in large quantities at low cost can lead to a new energy resource and provide energy self-sufficiency. A renewable method for generating H2 uses only sunlight and water, considerably reducing the costs and environmental impacts of fossil and nuclear fuels.
PEC technology affords real-time H2 production with water and solar energy. A PEC H2 production system integrates a semiconductor photoelectrode with an electrolyzer into a single, monolithic device, to produce H2 directly from water, using only sunlight. High performance solar PEC systems could offer the most efficient option for low cost, safe, lightweight H2 production to fuel the emerging fuel cell systems. Harvesting energy from the environment makes it possible to power micro fuel cells in real-time on board transportation vehicles or remote locations. The fuel cell becomes regenerative when the system is integrated with in-situ H2 production.
An integrated PEC cell offers the potential for cost effective, renewable hydrogen generation. Both n- and p-type semiconductors can be used for PEC splitting of water into H2 and O2. A PEC cell can provide about 30% efficiency advantage over a separate p/n photovoltaic (PV) system that is coupled to an electrolysis cell; it avoids the energy losses in the ohmic contact due to the mismatch of the Fermi levels and the band edges. So far, no single semiconductor has been identified, that can provide: (1) Correct energetics: bandgap, band edge overlap to drive the electrolysis reactions; (2) Fast charge transfer, and (3) Stability in an aqueous environment. Obstacles to direct photoelectrolysis of water are the lack of efficient light absorption when bandgap <2.0 eV, corrosion of the semiconductor, and unmatched energetics. The bandgaps of photochemically stable semiconductors are too large for efficient light absorption. Semiconductors with bandgaps in the optimal solar absorption range are typically thermo-dynamically unstable with respect to oxidation. The theoretical limit for water-splitting voltage is 1.23V. Practically, however, due to the existence of overpotentials at the electrolyte/electrode interfaces, the voltage needed is approximately 1.6V or greater. Thus a PV structure generates a voltage of approximately 1.6V or greater when operating under solar radiation.
A number of approaches have been tried, to overcome some of the obstacles to the direct splitting of water with a single electrode, including using: (a) simultaneously illuminated bi-photoelectrodes, (b) hybrid or bi-polar photoelectrode comprising a PEC cell and PV cell, and (c) Multiple-junction PEC cells. Several prior inventions and publications have disclosed designs for a variety of PEC cells. Most prior art PEC designs suffer from several shortcomings, including insufficient voltage to split water; the need for an external electrical bias for the electrolysis; corrosion in the electrolyte during operation for extended periods; expensive fabrication methods for the photoelectrodes; and low conversion efficiency. Therefore, there is a need to devise an efficient, stable, cost effective PEC cell with sufficient voltage to produce hydrogen from water, with a solar-to-hydrogen efficiency >10%.